Effect of agronomic measures on phosphorous loss via runoff in sloping croplands around Chaohu Lake

Eutrophication has been recognized as one of the main global environmental problems in areas with intensive agricultural production where there is nutrient export from farmlands to surface water bodies. Phosphorous (P) via runoff from farmland has been found to be one of the main contributors to water-quality degradation in Chaohu Lake. Thus, source control of P loss from farmlands was critical in remediation of eutrophication in Chaohu Lake. In order to test the efficiency and efficacy of different agronomic measures on reducing runoff, sediment and P loss, a long-term field runoff experiment was carried out in a sloping cropland around Chaohu Lake. Four treatments of agronomic measure were applied — 1) conventional tillage (CK), 2) plant hedgerow of Hemerocallis citrina (PH), 3) plant hedgerow with straw mulching (PHS) and 4) contour ridge (CR). Runoff volume, soil loss, concentrations of TP (total phosphorus), PP (particulate phosphorus), DTP (dissolved total phosphorus), D-Ortho-P (dissolved orthophosphate phosphorus) and DOP (dissolved organic phosphorus) as well as P loss via runoff were determined. The results for 2014-2015 showed that PH, PHS and CR treatments significantly reduced soil and water loss in the order as follows: PHS > PH > CR (P < 0.05). Also the reduction in sediment was great than that in runoff. Compared with CK, agronomic measures of PH, PHS and CR decreased runoff, respectively, by 23.5%, 36.5% and 19.7%, and reduced sediment, respectively, by 29.5%, 45.2% and 26.3%, showing significant effect on soil and water conservation (P < 0.05). The concentration of TP in runoff under CK was 0.612-1.220 mg·L^-1, in which PP was the predominant form, accounting for 71.5%-81.7% of the TP. In DTP, D-Ortho-P was the main form, accounting for 87.4%-90.7%, while DOP remained in low concentration. It was found that the three agronomic measures (PH, PHS and CR) significantly reduced PP and TP concentrations in runoff (P < 0.05), concurrently increased the concentrations of DTP and D-Ortho-P, but had no obvious effect on DOP concentration (P > 0.05). Obviously, TP concentrations in the current study were higher than 0.4 mg·L^-1, exceeding Class V level of the national environmental quality standards for surface water (GB3838—2002). The results implied that P loss via runoff from farmlands in Chaohu Lake region posed pollution risk if allowed to drain directly into the lake. Besides, annual P runoff loss from sloping croplands under CK was 0.706 kg·hm^-2, the equivalent of 0.98% of applied P fertilizer in the experiment. Compared with CK, PH, PHS and CR reduced annual P loss respectively by 38.4%, 53.8% and 33.4%. Another finding in the study was that the reduction in runoff volume and that in PP concentration were mainly responsible for the decline in P loss. In conclusion therefore, PH, PHS and CR significantly reduced soil, water and P loss via runoff in sloping farmlands in Chaohu Lake region. Among the treatments, PHS was the most effective. The results of this study provided more insight into the need for strategies to reduce agricultural non-point source pollution in Chaohu Lake region and other similar regions.